Method of producing a conductive layer of graphite on an insulating substratum



Dec. 31, 1957 v P. J. HESEMANS 2,318,355

METHOD OF PRODUCING A CONDUCTIVE LAYER OF GRAPHITE ON AN INSULATING SUBSTRATUM Filed NOV. 21, 1955 GLASS BULB GRAPH|TE,CONDUCTIVE LAYER SPRAYING BULB WITH GRAPHITE, ELECTROLYTE AND SYNTHETIC RESIN DRYING SPRAYED BULB IN HEATED OVEN WETTING WARM, DRY BULB WITH WARM WATER AT TEMPERATURE OF 50C TO 80C INVENTOR. Flg- 2 HERMAN PETER JACOB 1 BY HESEMANS AGENT United States Patent METHOD OF PRODUCING A CONDUCTIVE LAYER OF GRAPHITE ON AN INSULAT- ING SUBSTRATUM Herman Peter Jacob Hesemans, Eindhoven, Netherlands, assignor, by mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application November 21, 1955, Serial No. 548,266

3 Claims. (Cl. 117-226) This invention relates to a method of producing a conductive layer of graphite on an insulating surface, more particularly on the outer surface of glass articles such as, for example, bulbs of cathode-ray tubes.

It is customary to form such layers of graphite by applying a suspension of colloidal graphite in water stabilised by means of an electrolyte and comprising a little synthetic resin as a binder, onto an insulating surface and subsequently drying said layer in an oven. A stabilised suspension marketed particularly for this purpose is that of Acheson Colloids Ltd., 18 Pall Mall, London S. W. 1 under the name D68, CRT external wall coating compound. Such a suspension contains acetic acid as an electroylte, colloidal graphite and a synthetic resin and permits the production of conductive layers. However, such layers suffer from the disadvantage that the graphite is sensitive to moisture.

It has been found that such a graphite layer, which may have been baked for 12 minutes at 80 C., swells in a moist ambience and becomes detached from the substratum, sometimes with the formation of blisters. The present invention has for its object to render such a layer of graphite to moisture resistant.

The invention will be more fully described with reference to the drawings in which Fig. 1 shows a cathode ray tube coated according to the invention; and Fig. 2 is a flow sheet diagram of the necessary steps in carrying out the method of the invention.

In a method of producing a conductive layer of graphite, according to which colloidal graphite in the form in an electrolyte-stabilised suspension in water having a synthetic resin added to it is sprayed on an insulating surface and the layer of graphite is dried in an oven, this can be achieved, in accordance with the invention, by sprinkling the layer of graphite with warm water after drying.

Fig. 1 shows a typical glass cathode ray tube comprising a bulb 11, a faceplate 12, and a neck 13. An anode contact button 14 is mounted in the wall of the bulb 11 to provide means for connecting an electrical voltage source to the anode which is within the tube. The bulb 11 is provided with a conductive coating 16 which is commonly formed of a layer of colloidal graphite.

According to well-known procedures, the layer 16 is applied to the bulb 11 and is then baked dry in an oven. It has been found that sprinkling the graphite layer 16, preferably instantly after drying and before the layer ICC has cooled, with water having a temperature of 50 C. to 80 C. and preferably having a temperature of 55 C. for approximately to 20 sec., favourably 12 sec.,

, brings about such a modification of the layer as no longer to take up moisture. The layer may subsequently be wetted with water without swelling of the layer.

This may be explained by the fact that the hot water washes the electrolyte away with the result that the graphite separates as a flocculent precipitate and, together with its resin content, forms a watertight layer 16.

Fig. 2 shows a chart listing the essential steps of the process of the invention as outlined hereinabove.

If the layer 16 is to be provided on glass articles the temperature of the water should be such as to prevent cracking of the'glass or of the graphite layer due to sprinkling. The articles are preferably sprinkled upon leaving the oven. The oven has, in general, a temperature of to C. and it has been found that, for example a cathode-ray tube of the type shown in Fig. 1 and having a bulb 11 which consists of glass and is externally provided with such a layer 16 of graphite, and which has been dried in the oven for 10 minutes, allows of being sprayed with water of 55 C. instantly on leaving the oven and when the temperature of the glass bulb 11 is still 70 C. without cracking of the glass. When the sprinkling operation lasts more than 20 sec., there is a danger that the graphite layer 16 will chip ofi completely whereas sprinkling for less than 10 sec. will give poor results. The resistance of such graphite layers may remain, well below 500 9, measured between two points spaced 10 cms. from each other. The layer 16 is applied after the article, in the case under review the completely finished cathode-ray tube, has been evacuated and degased. Hence, in general, the method in accordance with the invention is the last treatment to which the article is subjected. Obviously, the warm water treatment of the layer may alternatively be effected in a manner different from spraying and sprinkling.

What is claimed is:

l. The method of applying a colloidal graphite layer to an insulating surface comprising the steps of forming a mixture of an electrolyte-stabilized suspension of colloidal graphite in water as one component and synthetic resin as another component of said mixture; applying said mixture to said insulating surface; heating said surface to dry the applied mixture; and, as soon as said mixture is dry and while said surface is still warm, wetting the dried layer with water having a temperature of approximately 50 C. to 80 C. for .a period of approximately ten to twenty seconds.

2. The process of claim 1 in which the temperature of the water with which said surface is wetted is approximately 55 C.

3. The process of claim 1 in which the step of wetting said surface is carried on for twelve seconds.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,818, 355 December 31, 1957 Herman Peter Jacob Hesemans It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

In the heading to the printed specification, between lines 9 and 10, insert the following Claims priority, application Netherlands November 29, 1954 column 1, line 37, after "graphite" strike out "to' Signed and sealed this 27th day of May 1958.

(SEAL) Attest:

KARL AXLINE ROBERT c. WATSON Attestlng Officer 5 Commissioner of Patents 

1. THE METHOD OF APPLYING A COLLOIDAL GRAPHITE LAYER TO AN INSULATING SURFACE COMPRISING THE STEPS OF FORMING A MIXTURE OF AN ELECTROLYTE-STABILIZED SUSPENSION OF COLLOIDAL GRAPHITE IN WATER AS ONE COMPONENT AND SYNTHETIC RESIN AS ANOTHER COMPONENT OF SAID MIXTURE; APPLYING SAID MIXTURE TO SAID INSULATING SURFACE; HEATING SAID SURFACE TO DRY THE APPLIED MIXTURE; AND, AS SOON AS SAID MIXTURE IS DRY AND WHILE SAID SURFACE IS STILL WARM, WETTING THE DRIED LAYER WITH WATER HAVING A TEMPERATURE OF APPROXIMATELY 50*C. TO 80*C. FOR A PERIOD OF APPROXIMATELY TEN TO TWENTY SECONDS. 